In the formation of integrated circuits, it is often necessary to etch an aperture in a silicon substrate. In particular, the trend toward packing more memory cells into a given chip area has led to the development of trench memory cells, which require deep, narrow apertures. In these cells the charge plate of the storage capacitor is formed by one or more polysilicon layers and one or more dielectric layers that are coated within a deep (8 to 12 micron) trench. The sidewalls of these deep trenches must be substantially parallel, having, if anything, a slight, positive taper such that the angle between the trench wall and the surface is between 88.degree. and 89.5.degree. to maximize cell retention. However, the taper can be varied between 84 and 89.5 to improve trench fill characteristics. Moreover, the trench walls must be as uniform as possible and the bottom of the trench should be as close to hemispherical as possible.
U.S. Pat. No. 4,717,448 (Cox et al.) discloses a process for forming deep trenches in a silicon substrate by reactive ion etching through a silicon oxide mask in a plasma formed from HCl, O.sub.2, and BCl.sub.3. Other RIE chemistries are known, although few have been applied to the problem of creating deep trenches.
Common problems encountered in creating deep trenches are (1) mask erosion; (2) "raking"; (3) bowing of the walls; (4) sloping walls; and (5) "dovetails" at the bottom of the trench. These various defects are illustrated in FIGS. 1 and 2 and are discussed below.
Referring to FIGS. 1 and 2 (undesirable features) and 3 (ideal features), one can compare the result of an ideal process to problems observed in previous processes. One of the common problems encountered in cutting deep trenches is bowing of the walls (102). The sidewalls of the trenches must be regular in that they must have a uniform slope (103); the slope of the sidewalls should vary minimally as a function of depth. The sidewalls should have a constant, near-vertical slope. A dovetail trench bottom (104), wherein the bottom of the trench has a convex, hemispherical shape, is undesirable. A concave, rounded trench bottom (105) is desired in order to optimize trench filling and eliminate void formation in the filler. A dovetailed trench bottom is difficult to reliably cover with layers to be subsequently deposited within the trench. Another phenomenon that causes problems is "raking." Raking is the staggered and irregular erosion of the silicon crystal near the mouth of the trench (106) (FIG. 2). It is readily observed by scanning electron microscopy. A further common problem is tapering of the mask edges, which in the ideal case should have a vertical profile
There is therefore a need for a process for creating deep, narrow, uniform trenches in a silicon substrate.